The bulk of the cancellations involve flights to and from Chicago and New York, SFO duty manager Doug Yakel said. He did not know how many planes would stay grounded Wednesday, saying the airport is in wait-and-see mode.

There aren’t many delayed flights, he said.

“It’s kind of an all or nothing deal,” Yakel said. “We’re either seeing flights that are straight out canceled or are leaving on schedule.”

The National Weather Service ranked the storm as the third-largest to hit Chicago since snow records began in 1886.

The blizzard accumulated 20.2 inches of snow at Chicago’s O’Hare International Airport as of 10:30 a.m. on Wednesday. Flights in and out of the airport won’t be possible at all until Thursday.